9 Factors That Can Affect the Accuracy of Your Temperatures

Despite what you may hear from people who don’t actually know anything about the Fertility Awareness Method, remembering to take your temperature each morning and check your cervical fluid throughout the day really isn’t all that hard or time-consuming, once you get the hang of it. But, just as there are certain factors that can lower the effectiveness of the Pill (such as taking the pill at a different time than normal, or being on certain medications, like antibiotics), there are also several factors that can affect the accuracy of your basal body temperatures, which may interfere with your estimation of your fertile window.

Any of the following nine factors can alter your basal body temperature, giving you a higher or lower reading than you would get otherwise. So, if any of these factors applies to you, pay close attention to your temperatures and make sure to mark them as ‘questionable’ in the app if they seem incorrect.

1. Restless sleep - You need at least 3 consecutive hours of uninterrupted sleep before waking up to get an accurate BBT. If you’re tossing and turning all night, or if you get up to go to the bathroom an hour before you actually wake up, your temperature will likely be affected. Mark it as questionable if this is the case.

2. Moving around before taking your temperature - Remember that you need to take your temperature as soon as you wake up, BEFORE getting out of bed or doing anything. ANYTHING. Even snuggling with your partner or just lying in bed awake for a few minutes. Again, if you don’t take your temperature right away, remember to mark it as questionable in the app.

3. Taking your temperature at a different time than usual - If you take your temperature much earlier than you normally do, your temperature will likely be lower than usual. If you take it later, it will be higher. Some women get around this by adjusting their temperatures .1 degrees F/.05 C for every half hour earlier or later that they wake up than normal - but if you’re considering this, be careful! Some women’s body temperatures change slower or faster than that. If you still want to do it, try it out over the course of a cycle or two and see if your temperatures appear consistent on the days that you need to adjust them.

If you work night shifts, all of the same rules for getting an accurate BBT still apply. Just try to be as consistent as possible with the time you’re taking your temp. If you work night shifts some days and day shifts on others, take your temperature first thing upon awakening from your longest period of sleep. Track your temperatures along with your cervical fluid and cervical position over the course of a few cycles, and see if you can notice a clear temperature shift. If so, you can continue to track your signs as normal, without having to worry about the varying wake-up times. If not, you’ll need to rely on cervical fluid and cervical position to learn when you’re fertile.

4. Drinking alcohol the night before - Drinking alcohol can raise your temperature the morning after. If you have several drinks, you’ll probably need to mark your temperature as questionable the next day. If you like to have, say, a glass of wine or two with dinner several evenings a week, keep track of your temperatures over the course of a few weeks, and on mornings after you’ve had a drink, make a note of it in the app. See if your temperatures appear consistent across your cycle. If so, it’s not necessary to mark your temperature as questionable every time you have a drink.

5. Stress - Stress can affect the body in lots of ways, and your basal body temperature is one of them. If you’re particularly stressed, you may notice a change in your temperatures. Mark them as questionable if they seem off.

6. Travel - If you travel to a different time zone, your temperatures may be affected, as your sleep schedule will likely be different. When traveling, wake up at the same time as you normally do (local time) and see if your temperatures appear consistent with the rest of your cycle. If so, you can continue to chart as normal. If not, mark them as ‘questionable’ until your body adjusts. Keep in mind that travel can act as a stressor on the body, which can sometimes delay ovulation.

7. Illness - Okay, this may be stating the obvious, but it goes without saying that being sick can throw off your temps. If sick, mark your temps as questionable or just wait until you’re better to take your basal temperatures.

8. Using an electric blanket or heating pad that you don’t normally use, or sleeping in a much warmer or colder room than normal - Some women’s bodies will be affected by the slightest change in room temperature, whereas others won’t be affected at all by a major change. Again, pay close attention to your temperature after nights that you sleep in a warmer or colder room than normal, or nights when you use an electric blanket or heading pad. See if the temperatures after those nights appear higher or lower than normal. If so, you’ll need to mark those temperatures as questionable.

9. Broken thermometer - Finally, if your temperatures WERE consistent but all of the sudden they seem off, you may want to consider purchasing another thermometer. Thermometers can sometimes go haywire without warning. Keep in mind that basal thermometers are generally more accurate than regular ones for the purpose of fertility tracking. (Want a highly accurate thermometer that's fast, comfortable in your mouth, and doesn't beep? Try Wink, a bluetooth basal thermometer that automatically syncs your temperatures and the time your temperature was taken to the Kindara app).

If you’re avoiding pregnancy and you need to mark one or several temps as questionable before you’ve confirmed ovulation with a sustained temperature shift, do not assume you’ve entered the infertile phase until you can see a clear pattern of post-ovulatory high temperatures above the coverline. If in doubt, abstain or use protection until your next cycle. If you need to mark a temperature as questionable while in your post-ovulatory (luteal) phase, you don’t need to do anything differently, since you’ve already confirmed that you’ve ovulated and are in your infertile phase.

If you’re trying to conceive and you need to mark one or several temperatures as questionable during your fertile window, make sure to pay especially close attention to your cervical fluid and cervical position during that time.